There are numerous instances in which it is desirable to find a ligand that specifically binds a receptor. To cite the most obvious examples, if the receptor is responsible for activation of a particular type of cell, ligands which bind the receptor may find therapeutic use in either activating or preventing the activation of the receptor, with a corresponding physiological effect on the cell. If the cell is contained in an animal or a plant, the effect may be felt by the entire organism. Thus, a very popular approach to designing new drugs rests on finding appropriate binding agents for these receptors.
Ligands that bind specific receptors can also find applications in analytical contexts. For example, antibodies are members of the generic class "receptors" and are useful components in immunoassay procedures. All of these procedures rely on the specific interaction between an antigen and an antibody; either partner may be the analyte.
In additions separation procedures and other processes with industrial application may take advantage of specific binding. To take a very straightforward illustration, an impurity may effectively be removed from a composition by treating the composition with a solid support to which is bound a "receptor" capable of binding the impurity to the relative exclusion of the other components of the composition, provided the affinity of the receptor for the impurity is sufficiently greater than for the desired components.
In all of the above cases, the amount of affinity that characterizes the specific binding and the degree of specificity required depends on the circumstances. Some applications are benefited by a relatively weak interaction, whereas others require a high affinity. Some applications are more demanding of specificity than others.
The obvious brute force method to find a ligand that will bind a receptor of interest is physically to test the capability of a large number of compounds which are potential ligands with respect to their ability to bind the target receptor. This method would no doubt eventually lead to finding a successful ligand in virtually every case but is clearly more time consuming and labor-intensive than would be desirable for practical utility. First, the receptor must be produced in some physical form that can be tested and sufficient quantities must be provided to test the range of compounds that are candidates. Second, if compounds are tested in just random order, a large quantity of receptor will be needed. This, especially in the case of cellular receptors, may be prohibitively expensive.
Several approaches have been suggested to minimize these difficulties. First, rather than testing compounds at random, a systematically varied panel of compounds could be used. Such systematically varied panels can conveniently be constructed by forming polymers from monomer units of predetermined characteristics. The most convenient such polymers are peptides, but polysaccharides, polynucleotides and the like could also be used. The parameters that are important and the manner of constructing such panels are described in U.S. Pat. Nos. 4,963,263 and 5,133,866, the contents of which are incorporated herein by reference.
In addition to, or instead of, using systematically varied panels of compounds as candidates, the screening itself can be conducted in such a way as to minimize the number of physical measurements that are required. For example, as set forth in U.S. Pat. No. 5,217,869, which is incorporated herein by reference, a reactivity profile for a ligand known to react with a target can be established by providing a standard panel of binding agents. The profile obtained characterizes this particular ligand known to bind the receptor. The candidate compounds can then be tested against the same panel to obtain their corresponding profiles. When a corresponding profile matches that of a ligand known to be a successful binder to the target, the compound which generated the matching profile will have a high probability of binding the receptor. In an alternative, inverse image panels are prepared with varying characteristics, and profiles obtained for the receptor and ligand against opposite panels are matched.
Various other technologies are directed to methods to improve the ease with which the physical binding of receptor to candidate ligand can be measured, such as the use of robotics, fluorescence detection of reactivity, physical arrangements of the panels, and so forth.
Other methods which seek to find specific binding pair members include computer based methods such as three dimensional database searching, x-ray crystallography, molecular modeling, and the like. Other methods employ antibodies as surrogate targets or simply rely on the behavior of the compound with respect to related target receptors. For example, the behavior of a compound as an inhibitor of a particular serine protease, or of a number of serine proteases, might lead one to assume that it will be a useful inhibitor of an additional serine protease for which its inhibition activity has not yet been determined. The validity of this last mentioned method relies on the similarity of the serine proteases that are the "reference receptors" for which the binding characteristics of the test compound are known to the target receptor (serine protease) for which the binding characteristics are not known.
The present invention provides another method to match ligand with a target receptor. It is especially helpful when limited supplies of the target receptor are available. The invention method is especially useful in drug design projects where the target has never been fully purified, is unstable or otherwise not available in adequate quantities for large-scale screening, or when the assay procedure for the target is complex and costly. Further, the method minimizes consumption of receptor in a program of screening against many potential ligands.